Cash, Cash Equivalents and Investment Debt Securities (Cash, Cash Equivalents and Investment Debt Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	$ 63,377
Cash and cash equivalents, Fair Value	63,377
Investment Debt Securities Abstract
Total investments, Amortized Cost	648,047	$ 393,617
Total cash and cash equivalents and investment debt securities, Amortized Cost	711,424	436,865
Total investments, Gross Unrealized Gains	1,050	32
Total cash and cash equivalents and investment debt securities, Gross Unrealized Gains	1,050	32
Total investments, Gross Unrealized Losses	(103)	(737)
Total cash and cash equivalents and investment debt securities, Gross Unrealized Losses	(103)	(737)
Total investments, Fair Value	648,994	392,912
Total Investments, Fair Value	648,994
Total cash and cash equivalents and investment debt securities, Fair Value	712,371	436,160
Commercial Paper [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	70,009	34,353
Total investments, Gross Unrealized Gains	62
Total investments, Gross Unrealized Losses	(8)	(26)
Total investments, Fair Value	70,063	34,327
Corporate Debt Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	574,046	349,854
Total investments, Gross Unrealized Gains	984	27
Total investments, Gross Unrealized Losses	(95)	(704)
Total investments, Fair Value	574,935	349,177
US Government Agencies Debt Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	3,992	9,410
Total investments, Gross Unrealized Gains	4	5
Total investments, Gross Unrealized Losses		(7)
Total investments, Fair Value	3,996	9,408
Cash and Money Market Funds [Member]
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	63,377	43,248
Cash and cash equivalents, Fair Value	$ 63,377	$ 43,248